The present invention relates generally to field of control circuits for discharge lamps. More particular, the invention relates to control circuits to arc or discharge lamp ballast technology that maintains a constant light output as the lamp ages.
It is well known to utilize the ballast in the gas discharge lamps to obtain high energy efficiency and long lamp life. Several prior electronic ballast and network for gas discharge lamps provide circuits to control the illumination intensity of the lamp. One such known method is provided in U.S. Pat. No. 5,315,214 to Lesea, which shows a circuit controlling the intensity of the lamp by controlling the current passing through the lamp. Also, this circuit shuts off the lamp circuit when the lamp voltage exceeds a preselected threshold. One of the disadvantages of this prior circuit is that because the lamp current remains constant, the illumination intensity of the lamp will vary with impedance changes caused by the age of the lamp. Furthermore, by sensing the lamp voltage to shut down the circuit, this lamp circuit does not protect the lamp from circumstances when the lamp current remains constant and the lamp voltage rises thus causing excess power to dissipate into the lamp.
Another known method is provided in U.S. Pat. No. 6,011,363 to Allison. This patent shows a constant wattage electronic ballast circuit which produces a voltage feedback signal from the lamp. The voltage feedback signal is processed to produce a processed signal which is further summed up with the current feedback signal from the lamp to create a summed signal. The lamp current is automatically adjusted in response to the difference between the summed signal and a reference signal. The power of the lamp is controlled by regulating the current of the lamp. One problem with regulating only the lamp current is that the light output of the lamp is more closely related to the arc power of the lamp than to the lamp current. Lamp voltage, however, is dependent on the temperature of the lamp. Therefore, if only current is regulated, the arc power, and hence, light output, will vary with the temperature of the lamp. Further, when only lamp current is regulated, increases in the impedance of the lamp caused by aging results in increased power dissipation. As a result of these factors, portions of the gas discharge lamp reach excessive temperatures. This can present a dangerous fire hazard and can cause the glass envelope of the lamp to shatter, posing an immediate safety hazard for persons in the vicinity of the lamp.
Another known method is provided in U.S. Pat. No. 6,232,727 to Chee et al. which shows a circuit controlling the illumination intensity of the lamp by regulating and/or controlling the arc power delivered to the lamp network. The level of the power provided to the lamp is sensed by providing a regulated voltage to a lamp network or by sensing current leaving the lamp network. The power provided to the lamp network is controlled by adjusting the frequency of the voltage provided to the lamp network. However, again, only by controlling the voltage provided to the lamp network, the power provided to the lamp is controlled such that the user selected illumination intensity output for the lamp is maintained.
Therefore, a need exists to provide a control circuit for a gas discharge lamp that overcomes these disadvantages and provide a substantially constant light output throughout the life of the lamp.
The present invention provides a method and apparatus for controlling the operation of an arc ballast lamp. The invention includes a ballast circuit for providing power to the lamp, a monitoring circuit for receiving voltage signal and the current signal drawn by the lamp, a dividing circuit for obtaining a signal representative of the lamp resistance based on the lamp""s voltage and current signal, and a comparator circuit for adjusting the power provided to the lamp in response to the signal received from the dividing circuit, to maintain a substantially constant light output from the lamp.